DC (Direct Current)-to-DC converters may be found in many electronic devices. For example, DC-to-DC converters are often found in PDAs (Personal Digital Assistant), cellular phones and laptop computers. These electronic devices often contain several sub-circuits with different voltage level requirements from that supplied by a battery or an external supply. A DC-to-DC converter converts a source of direct current from one voltage level to another voltage level in order to meet the voltage levels required by sub-circuits.
One method of providing DC-to-DC conversion is through the use of a linear regulator. However, a linear regulator may dissipate too much heat for devices such as laptop computers and cellular phones.
DC-to-DC converters convert one DC voltage to another by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components (e.g. inductors, transformers) or electric field storage components (i.e. capacitors) or a combination of both. This conversion method is more power efficient (often 75% to 98%) than a linear regulator, for example. This efficiency is beneficial to increasing the running time of battery operated devices.
Among the different DC-to-DC converter topologies, the LLC (Inductor Inductor Capacitor) converter has attracted substantial attention recently because of its high efficiency, smooth waveform and high power density. The high efficiency of the LLC converter is due to the use of zero voltage switching (ZVS). ZVS reduces switching loss which in turn improves the efficiency of the LLC converter.
An LLC converter operates in a resonant mode. During a resonant mode of operation, signals having a fixed duty cycle (approximately 50%) and a variable period drive powers switches. Power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are typically used as power switches. The start up time of an LLC converter is quite critical. At start up, the output capacitors of an LLC converter are usually discharged. When an LLC converter starts to charge a discharged output capacitor, the instantaneous current or surge current drawn through a power MOSFET can be too great and cause the power MOSFET to stop functioning. Gradually charging output capacitors during start up of an LLC converter can prevent a power MOSFET from being rendered inoperable.